The invention is particularly applicable to inductively heating and quench hardening the cylindrical toothed surface of an axially elongated gear and will be described with particularly reference thereto. However, the invention has broader applications and may be used for inductively heating and quench hardening other elongated workpiece with a cylindrical surface generally concentric with a central axis and having an undulating cross-sectional outline, such as the outline defined by gear teeth. The invention is also particularly applicable to induction heating and quench hardening of the internal surface of an internal planetary ring gear, especially an internal planetary ring gear which is closed at one end and thus not able to be shifted directly between axially aligned inductor coils.
It is desirable to harden the toothed surface of a gear to enable that surface to withstand the wear and contact forces exerted during operation of a high power transmitting gear train. The surfaces to be hardened for resistance to contact forces exerted by intermeshing of other gears include the connecting surfaces between the gear teeth as well as the flanks and tips of the gear teeth themselves. The body of the gear beneath the hardened surface should remain relatively soft to provide strength and ductility to the gear structure. Ideally, the gear would have a hardness pattern extending to a uniform and shallow depth across the entire hardened surface to provide the resistance to surface abrasion associated with hardening while at the same time preserving the strength of the underlying material by avoiding the brittleness associated with hardening at the body of the gear beneath the gear teeth surfaces.
Previous methods of using induction heating to harden gear teeth have had limited success in obtaining the desired pattern of hardness extending to a shallow depth uniformly across the gear teeth surfaces. The material to be hardened must be raised above a transformation temperature and then quickly cooled by liquid quenching to induce hardening. Factors affecting the resulting hardness pattern include the depth to which the material is heated, the degree to which the heated temperature exceeds the transformation temperature, and the rate of cooling. A circular inductor coil closely spaced from the undulating gear teeth surface generally exposes the radially outer most regions of the gear teeth to a greater degree of induction heating than the connecting regions between the teeth, with the result that the temperature and depth of heating is correspondingly greater at the outer regions of the gear teeth. The differently heated regions will then be cooled at differing rates in the liquid quenching process, with the result that the hardness pattern developed thereby will be uneven across the gear teeth, with excessive hardening to depths beneath the gear teeth surfaces and with insufficient hardening at the connecting surfaces between the gear teeth. Accordingly, in order to successfully harden gear teeth by induction heating it is necessary to heat the gear to a preselected temperature uniformly to a controlled depth immediately before quench hardening.
A method of providing a hardness pattern to a uniform depth across the surface of gear teeth is shown in U.S. Pat. No. 4,675,488, also assigned to the present assignee and incorporated herein by reference. In that method a gear is closely received within an encircling inductor coil having an axial length or height corresponding to the axial length of the gear. The inductor coil is then energized with an alternating current having a frequency of less than about 50 KHz for a short period of time to preheat the gear. Heating depth in induction heating is inversely proportional to the current frequency at the inductor coil, whereby preheating at a relatively low frequency penetrates the gear to the base or roots of the teeth to heat a circular band extending around the gear beneath the gear teeth. This internal band is heated to an elevated temperature below the quench hardening transformation temperature of the metal material. A short time delay follows the preheating step to allow heat energy in the teeth to dissipate and thus to permit concentration of a high temperature and energy level within the band adjacent the roots of the teeth. Preferably, a second low frequency preheating step follows the time delay to further heat the underlying band and also to preliminarily heat the gear teeth surfaces to an elevated temperature. In this manner, the relatively low frequency preheating steps store and concentrate a high energy and high temperature at the internal band extending circumferentially through all of the root portions of the gear teeth. This internal band is at a higher temperature than the teeth themselves and is at a temperature substantially greater than the temperature of the underlying core of the gear. This preheated temperature profile is very dynamic and unstable, so the preheated gear is then immediately transferred into a second encircling inductor coil which is immediately energized with a high frequency current of greater than about 100 KHz for a short period of time. The applied high frequency current elevates the gear temperature above the quench hardening transformation temperature only to a shallow depth beneath the gear teeth surfaces. This shallow depth heating above the quench hardening transformation temperature is uniformly distributed throughout the gear teeth flanks and the connecting root portions since the internally heated band provides preheat energy at the connecting roots to enable them to attain the elevated final temperature along with the tooth flanks and tips more closely spaced from the inductor coil, and further inhibits an otherwise rapid conduction of heat from the tooth surfaces into the teeth and the core of the gear. The finally heated temperature profile is also highly unstable and dynamic and therefore is immediately followed by liquid quenching of the heated surfaces to bring the shallow depth of high temperature material below the transformation temperature whereby a uniform hardness pattern is provided at a corresponding shallow depth across the entire gear teeth surface.
The above described method successfully hardens gear teeth uniformly on their outwardly facing surfaces without causing brittleness due to excessive heating depth, and without leaving soft unhardened surfaces between the gear teeth due to lower temperatures before quench hardening. However, that patented method does not ideally enable hardening of gear teeth by means of induction heating and suffers from several disadvantages. The gears first move axial into an encircling induction heating coil for audio frequency heating during the two preheating cycles, and is then shifted axially into a second induction heating coil for final heating at the higher radio frequency current. The two axially spaced induction coils must each have an axially length exceeding the axial length of the gear so that the total gear will be heated at one time during both preheating and final heating. Since the temperature profiles obtained during practice of the method are highly unstable and dynamic, heating must occur very rapidly. The requirements for axial length and rapid heating ability at the inductor coils impose a requirement for a high power density over the elongated area being heated, which in turn imposes the requirement of a substantially high power rating for each inductor coil power supply. It is well known that as the rating of a power supply increases, especially an oscillator as used for radio frequencies above 100 KHz, the cost of the power supply drastically increases. For this reason, inductively heating gear teeth in accordance with the above described patented method is relatively expensive and sometimes impractical when the gears to be hardened are large, either in diameter or in axial length or height.
Another method previously patented by the present assignee overcomes many of the disadvantages associated with the above described method, yet is still not ideally efficient. That method, disclosed in U.S. Pat. No. 4,757,170, incorporated herein by reference, involves moving the gear progressively through two axially aligned inductor coils to preheat and then to finally heat the gear teeth surface progressively along its axial length, as opposed to heating the entire length of the gear distinctly and separately in the two coils as in the method described above. Progressive scanning along the length of the gear permits the induction coils to be of a narrower length or axial height, because at any one time the high power density required to impart the desired temperature profile needs only to be provided at a narrow band extending around the gear and corresponding to the width of the relatively axially moving inductor coil. Since heating is done progressively and simultaneously on the gear surface, the high power density results from the reduced size of the heating bands for both preheating and final heating, and does not require a substantially high power rating as is required to incrementally preheat and final heat the entire length of the gear. Accordingly, the latter described method overcomes the disadvantages of cost, efficiency, and workpiece length associated with the former described method.
However, there are still several disadvantages associated with practice of the latter method. The apparatus required to controllably move the workpiece through two separate coils is complex and expensive. Furthermore, simultaneous movement of the gear through the two coils necessarily imposes the same axial scanning velocity on both the preheating and the final heating processes. Controlled variations in the heating parameters employed in these two distinct heating processes are thus severely limited. Furthermore, a disadvantage attendant to practice of both of the aforementioned methods of induction heating and hardening of gear teeth is the fact that internal planetary ring gears which are often closed at one end cannot be coaxially shifted between or passed to separate inductor coils, nor can external gears with outwardly protruding flanges, shoulders, or the like. These patented methods are therefore limited in their applications to gears having external gear teeth and no substantial radial protrusions.